1. Field of the Invention
The invention relates to a fuel cell system that has a secondary cell, and to a control method for the fuel cell system.
2. Description of the Related Art
Known control methods for a fuel cell system that has a battery (secondary cell) include, for example, a control method described in Japanese Patent Application Publication No. 2007-5038 (JP-A-2007-5038). A control unit of the fuel cell system described in Japanese Patent Application Publication No. 2007-5038 (JP-A-2007-5038) performs a control of firstly restricting the output voltage Vfc of the fuel cell with reference to an oxidation-reduction potential, and using the battery to compensate for a power that corresponds to the restriction in output voltage, even when the demanded system electric power Wreq is gradually increasing. After that, the control unit keeps the output voltage of the fuel cell at or below the oxidation-reduction potential and thus continues electricity generation by the fuel cell even in the case the need for the electricity generation disappears due to a drop of the amount of accelerator operation, or the like. The control unit stores the surplus electric power provided in the foregoing case into the secondary cell.
The fuel cell usually degrades and its characteristics and the state of the fuel cell change as it is used for a long time. In the foregoing related-art technology, how to perform the electric charging control of the battery (secondary cell) in association with changes in the characteristics and the state of the fuel cell is not fully considered. Besides, in the case where the fuel cell has not degraded but the temperature of the fuel cell or the battery is low, it is preferable to change the operation method to a method that is optimum for raising the temperature of the fuel cell or the battery. However, the devising in that respect in the related-art technology is actually not sufficient.